David Herbert Donald — American Historian born on October 01, 1920, died on May 17, 2009

David Herbert Donald was an American historian, best known for his acclaimed 1995 biography of Abraham Lincoln. He twice won the Pulitzer Prize for Biography for earlier works; he published more than 30 books on United States political and literary figures and the history of the American South... (wikipedia)

Lincoln had no such person that he could talk with. Often, as a result, he debated with himself, and he would draw up a kind of list of the pros and cons of an argument, and carefully figure them out, and he might test them in public.
The big biography of Lincoln necessarily had to do so much with his political career, his ambitions, his accomplishments in public, with less time to spend on his private life, his inner life, and I thought this might be a way of getting at that.
If you think about it, the historian's task is like that of the detective.
In Lincoln's day a President's religion was a very private affair. There were no public prayer meetings, no attempts to woo the Religious Right. Few of Lincoln's countrymen knew anything at all of his religious beliefs.
The more I have studied Lincoln, the more I have followed his thought processes, the more I am convinced that he understood leadership better than any other American president.